In an axial flow gas turbine engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot gases of combustion are passed through turbines mounted on the same shaft. The flow of combustion gas turns the turbine, and the rotating turbine extracts energy from the flow of the combustion gas which can be converted to electricity and turns the shaft that powers the compressor. The expended exhaust gases may be exhausted to the atmosphere.
The compressor blades used in turbine applications for land-based turbines are usually made of 400 Series stainless steel. These steels are usually sufficiently strong for the purpose, but are subject to erosion and corrosion mechanisms, by nature of their operation which involves compressing large volumes of air, which air may include contaminants. Contaminants found in air include, but are not limited to, unwanted oxides, carbon dioxide, chlorides (salt), SO2, SO3, sulfate-based salts, sulfides dirt, and organic impurities. Dirt includes sand, volcanic ash, fly ash, cement, dust and even substrate impurities.
To improve the erosion and abrasion behavior of compressor blades, an erosion-resistant or abrasion-resistant coating may be applied over the airfoil portion of the blade, which is exposed to the large volumes of air on which is compressed. Frequently, these coatings may be limited to the airfoil leading edge and tip. The coatings are usually cobalt-based alloys having some tungsten (W) and carbon (C) to facilitate the formation of WC/Co particles that enhance the erosion and abrasion resistance of the coating.
These stainless steel blades coated with abrasion-resistant cobalt-based alloys still may be subject to attack simply as a result of the environment in which they are used. The blades may be subject to galvanic corrosion when used in an environment which may be acidic. This may result from the gas turbine being placed in an ocean atmosphere or even in proximity to a nearby chemical plant, such as a chemical petroleum plant or refinery. The chemicals from these environments as well as the other contaminants found in air may combine with moisture in the air to produce a corrosive environment. Also, many modern gas turbine engines may introduce moisture by use of an on-line wash system that provides operational cleaning of the blades, or fogging and/or evaporator systems to enhance compressor efficiency. These systems may combine with contaminants or environmental chemicals to produce a corrosive environment. In addition, green initiatives suggest exhaust gas recirculation of at least of portion of exhaust gases captured by the exhaust gas recapture system to reduce CO2 emissions. Such exhaust gas recapture systems include stoichiometric—cooled exhaust gas recirculation, or SEGR. The recirculation of exhaust gas by introducing it into combustion air will increase the concentration of chemicals forming a corrosive environment to which the compressor blades are exposed. Current coatings and materials combinations can be vulnerable, especially when the SEGR gas path contains contaminants such as SO2.
While the stainless steel compressor blades coated with a cobalt-based abrasion-resistant alloy exhibit good erosion and abrasion resistance, compressor blades, such as compressor blades formed from a martensitic stainless steel such as GTD-450 and coated with a cobalt-based coating such as STELLITE® are subject to galvanic corrosion in a corrosive environment, which is even more severe in a SEGR mode. In such an environment, the compressor blades may be subject to crevice corrosion or pitting. The cobalt-based coating tends to be cathodic with respect to the compressor blade base material. As a result, galvanic corrosion will occur in the presence of the corrosive media as concentration cells are set up in crevices or recesses where deposits of contamination or corrosion product are trapped and become stagnant. Crevice corrosion may occur in these regions. Pitting corrosion also may occur as small pits or holes form in the coating where deposits of contamination form. These pits may initially develop from impacts with foreign objects and contaminants drawn into the gas turbine. These impacts dent the coating and enable corrosive media to collect. What is needed is a coating for use over stainless steel compressor blades in which the coating maintains the characteristics of erosion and abrasion resistance, and protect the base metal from galvanic or dissimilar metal corrosion in the case of ingress of corrosive species through a breach in the coating caused by erosion or other damage.